Due to serious explosions which have occurred in kraft chemical recovery furnaces, studies have been conducted in order to analyze the mechanics of these explosions with a view toward developing methods by which they can be prevented. These studies indicate that violent noncombustible physical explosions can result when a quantity of water or water solution of chemicals contacts the molten kraft smelt which collects on and drains from the furnace floor. The water may become available for such explosive interaction with the molten smelt under conditions such as might result from a tube leak. These same physical explosions can occur upon contact between a molten metal and water.
Although the present invention is not to be limited by any particular theory for the cause of the physical explosions of the smelt-water reaction type, one possible explanation for this type of explosion is suggested in "A New Theory To Explain Physical Explosions" by Wharton Nelson, TAPPI, March 1973, Vol. 56, No. 3, pp 121-125. This theory indicates that the initial steam film forming on molten smelt contact with liquid water soon collapses and is replaced by a thin shell of superheated water. High superheat is possible since liquid-liquid surface contact presents few nucleation sites. This thin layer reaches its limit of superheat and spontaneously explodes; i.e., converts to steam. This sharp but weak triggering (blasting cap-like) explosion causes an extremely rapid heat transfer due to the finely divided molten smelt particles created which are now traveling at high velocity through bulk liquid water. These ideal heat transfer conditions result in large volumes of water being rapidly converted to steam with a consequent 1700-fold multiplication of volume upon phase change.
This sudden creation of gas (steam) constitutes a physical explosion phenomenon which occurs only when two liquid materials at widely different temperatures contact each other. Physical type explosions contrast with the combustible type of explosion which produces rapidly expanding gases by a highly exothermic chemical reaction. Since the two explosion causing mechanisms are different, prevention methods would necessarily be different also. For example, while merely inerting a furnace atmosphere with gases such as water vapor and/or carbon dioxide would prevent a combustible explosion, such a technique would not counteract a physical explosion which can occur under water or in a 100 percent nitrogen atmosphere. Several different techniques have been developed for preventing such explosions including those described in U.S. Pat. Nos. 3,447,895 and 3,615,175.